One thing you haven't mentioned is if there's any tweak in the body. Since the body is welded to the frame it should act very much like a unibody. If the frame was twisted after the body was welded on, there would have to be some deflection of the sheet metal. Since you didn't mention any I'll guess that the car was put together wrong/distorted in the first place.

Tritto on Dave and Al's comments about fabbing a new frame.